Problem: Which of these numbers is prime? ${15,\ 25,\ 47,\ 51,\ 70}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 25 are 1, 5, and 25. The factors of 47 are 1 and 47. The factors of 51 are 1, 3, 17, and 51. The factors of 70 are 1, 2, 5, 7, 10, 14, 35, and 70. Thus, 47 is a prime number.